Long term measurements of VOC exchanges above a maize field at Lonzée (Belgium)

For the last decades, VOC had arisen scientifict interest due to their important role in the atmospheric chemistry and their final impact on air pollution and climate change. Terrestrial ecosystems being the main VOC source, evaluation of current and future biogenic VOC emissions through VOC exchange modeling is thus necessary to better estimate future climate and assess future air pollution risks.
BVOC exchanges depend on edaphic variables and are plant species specific. Therefore, their modeling and global budget evaluation requires a comprehensive understanding of production and exchange dynamics under a wide panel of climatic conditions and ecosystems, which necesserily implies BVOC exchange measurements under varied conditions.
In that perspective, forest and non pastured grasslands have been largely studied for the last decade, but knowledge about BVOC fluxes from croplands remains still scarce. As a consequence, crop species-specific standard emissions that feed bottom-up BVOC emission models are still often assigned to a default value that is in addition kept constant for the entire growth season, although recent research has shown that plant phenology, acclimation and stress can drastically influence BVOC emissions.
To help filling this knowledge gap, we run a project that aims to study VOC fluxes from two major croplands, maize (2nd most important culture worldwide) and winter wheat (1st most important culture worldwide), and a pastured grassland. We present here a specific study focussing on the VOC exchanges between a maize field and the atmosphere. VOC fluxes were measured at ecosystem-scale during the whole 2012 growing season using the eddy covariance by mass-scaning technique with a proton-transfer-reaction mass spectrometer. Together with VOC fluxes, we also recorded a wide set of ancillary data including CO2 fluxes, meteorological variables and biomass evolution. As far as we know, we are the first study dealing with BVOC measurements on maize at ecosystem scale and spanning all the phenological stages of the crop.
Although first results show half-hourly bidirectionnal exchanges among all the preselected compounds, in average methanol is the greatest emitted VOC, followed by green leaf volatiles. Acetic acid and acetaldehyde are the greatest taken up VOC. Small isoprene and monoterpene fluxes are also observed. A diurnal pattern is found for all those VOC, with greater emission/uptake during the day, suggesting a flux dependence on environmental parameters. Influence of environmental controls, biomass evolution (including growth primary production) and phenology on fluxes is currently under investigation.
Our research allows to quantify BVOC exchanges by a maize field throughout a whole growing season. Hence, obtained results will refine the understanding of the BVOC exchanges mechanisms by including both environmental and phenological parameters. Such results are expected to be very useful for BVOC modeling, especially for oxygenated compounds such as methanol.